Data represents a significant asset for many entities. Consequently, data loss, whether accidental or caused by malicious activity, can be costly in terms of wasted manpower, loss of goodwill from customers, loss of time and potential legal liability. To ensure proper protection of data for business and legal purposes, many entities back up data to a physical storage media such as magnetic tapes or optical disks. Traditionally, backup would occur at each machine controlled by an entity. As the sophistication of network technology increased, many entities turned to enterprise level backup in which data from multiple machines on a network is backed up to a remote library. Typically, a library includes a variety of components which include a plurality of media for data storage, e.g. multiple magnetic tapes. Centralized data backup has the advantage of increased volume, efficiency, and redundancy.
In many systems, the data to be backed up and backup commands are sent over a network from multiple machines on the network to a library. In many instances, the data to be backed up and the backup commands are routed to the library through a switch.
One example of a library commonly used in enterprise backup systems is a magnetic tape library. A magnetic tape library can comprise components such as tape cartridges (containing magnetic tape), robots, tape slots and tape drives. A typical magnetic tape library contains multiple cartridge slots in which tape cartridges can be stored. Tape cartridges, commonly referred to as tapes, are physically moved between cartridge slots and tape drives by a robot. The robot is controlled by commands received from the host devices on the network. When specific data is required, a host device determines which cartridge slot contains the tape cartridge that holds the desired data. The host device then transmits a move-element command to the robot and the robot moves the tape cartridge to a tape drive which reads the desired data from the tape cartridge.
In a SCSI tape library, devices that are part of the library are typically addressed by target number. Thus, each drive and robot of a tape library typically has a target number. Cartridge slots, on the other hand, are addressed by element numbers that are used by the robot to locate the slots. Because the robot also places tape cartridges in the drives, each drive is also associated with an element number.
Components of a library are subject to wear and other forms of degradation which may degrade individual library component(s) to a point where the degraded component(s) can no longer be reliably used. Degradation of library components may render a library unreliable and prone to data loss, diminishing the library's usefulness as a backup device. The failing of even one library component can cause the loss of valuable data. Furthermore, the process of determining which library component is failing and rendering the library unreliable can be time consuming, uncertain and expensive. Networks and other equipment external to the library may also affect the performance of the library. For instance, if data is being backed up from a remote location, transport delays and interruptions along the transmission path may cause the data to arrive relatively slowly (or not at all) at the library. While the library may store the data just as fast as it arrives, the performance of the library suffers because the storage rate in these circumstances is nonetheless slow.